Washington (CNN) -- The U.S. Supreme Court on Monday reinstated the criminal conviction of a sex trafficker known as the "S&M Svengali."

The case gave the justices a rare visit to the shadowy world of sadomasochism and sex slavery.

In a 7-1 ruling, the high court allowed the original conviction of Glenn Marcus to stand. He had been sentenced to nine years in prison for the sexual abuse, physical mutilation and psychological humiliation of a woman who had agreed to be photographed as his "sex slave."

A federal appeals court in New York had dismissed the entire conviction, saying some of the offenses occurred before the 2000 Trafficking Victims Protection Act, which was used to prosecute Marcus.

But Justice Stephen Breyer said the procedural violations in this case were not so severe to justify throwing out the entire case since some of the offenses clearly occurred after the law was passed.

"Given the tiny risk that the jury would have based its conviction upon those few pre-enactment days alone," Breyer wrote, "a refusal to recognize such an error as a 'plain error,' [and to set aside the verdict] is most unlikely to cast serious doubt on the fairness, integrity, or public reputation of the judicial system."

Establishing a clear standard for "plain error" review when setting aside convictions has been a particularly tricky area of law for the Supreme Court in recent years.

According to the trial record, Marcus ran a Web site that featured photos he had taken of women who acted as "sex slaves" and were subjected to varying levels of physical abuse.

The woman at the center of the case -- identified only as "Jodi" -- had met the defendant in 1998 and agreed to participate in his commercial activities.

At issue was whether Marcus took the relationship too far and held Jodi against her wishes. Federal prosecutors claim he manipulated and forced the woman to undergo the punishment, and then to write about it for the man's Web site.

The incidents took place at various locations -- including in New York and Maryland -- between January 1999 and October 2001. Some were before the law was passed; some were after.

Attorneys for Marcus said the relationship was consensual, even enjoyable, that Jodi had signed an employment contract and was provided for through the for-profit Web site, which had paying members and advertising. They also said that while the public may find the details unsettling, it was done in the privacy of homes.

The woman testified she felt like a prisoner and she could not escape her situation. Her head was shaved and the word "slave" was written on her stomach by Marcus with a knife. She claimed she was whipped regularly, hung by her arms from posts and subjected to a range of humiliating poses.

The 2nd U.S. Circuit Court of Appeals concluded that Jodi never benefited financially since Marcus had kept profits from the Web site. Other women along the East Coast also were involved in Marcus' master-slave business.

Justice Sonia Sotomayor -- who was on that appeals court -- did not take part in the high court's review of the case.

Like her colleagues on the appeals bench, Sotomayor said at the time of the appeals court ruling that prior precedents required the conviction be set aside, but she conceded the Supreme Court might view the issue differently.

She noted the justices had, in some cases, found that lower court errors in the application of the law "do not seriously affect the fairness, integrity or public reputation of the judicial proceedings."

And that is exactly what all but one of her high court colleagues concluded.

Justice John Paul Stevens dissented, saying Marcus' "substantial rights" were undermined. Stevens said he worried his colleagues "have trapped the appellate courts in an analytic maze that, I have increasingly come to believe, is more likely to frustrate than to facilitate sound decisionmaking."